The conventional telephone circuit is capable of providing the user with the caller identification service in conjunction with a telephone system company, such as AT&T. When the user is on the telephone, the hybrid circuit of the user's telephone is capable of receiving the CAS signal transmitted from the exchange at such time when a third person calls. The CAS signal is then transmitted to the CAS detector circuit for identification of the telephone number of the third person. However, the internal impedance of the hybrid circuit is not intended to cooperate with the frequency of the CAS signal, thereby unabling the internal impedance of the hybrid circuit to match very well with the impedance of both tip and ring of the telephone line. In addition to the CAS signal, a small portion of the voice signal which come from the microphone of a near-end telephone set can also appear on the CAS detector circuit. When the CAS signal is transmitted by the exchange, the CAS detector circuit is incapable of identifying the CAS signal if the voice frequency received by the microphone of the near-end telephone set is close to the CAS signal frequency (2130 Hz or 2750 Hz).